Steering wheels control buttons

I wonder how can I connect my steering wheels control buttons to operate my Becker (Harman/Kardon) radio-cd player.

My point is to find the way to use the i-Bus protocol used by BMW since the car is a BMW Z3 (model 1999) and the radio-cd player is the Becker MexicoPro CD.

In fact my car's steering wheel did not have control buttons. However, I retrofitted them since the same steering wheel (unfortynatelly not the same in its base) was used in another BMW model wich had the control buttons on it and they were operating through i-Bus protocol.

In the picture attached you can see my steering wheel. I do have the wiring behind them but I do not know what to do with them.